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gettimeofday.2

GETTIMEOFDAY

Section: Linux Programmer's Manual (2)
Updated: 2009-03-25
Index Return to Main Contents
 

NAME

gettimeofday, settimeofday - get / set time  

SYNOPSIS

#include <sys/time.h>

int gettimeofday(struct timeval *tv, struct timezone *tz);

int settimeofday(const struct timeval *tv, const struct timezone *tz);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

settimeofday(): _BSD_SOURCE  

DESCRIPTION

The functions gettimeofday() and settimeofday() can get and set the time as well as a timezone. The tv argument is a struct timeval (as specified in <sys/time.h>):

struct timeval {
    time_t      tv_sec;     /* seconds */
    suseconds_t tv_usec;    /* microseconds */
};

and gives the number of seconds and microseconds since the Epoch (see time(2)). The tz argument is a struct timezone:

struct timezone {
    int tz_minuteswest;     /* minutes west of Greenwich */
    int tz_dsttime;         /* type of DST correction */
};

If either tv or tz is NULL, the corresponding structure is not set or returned.

The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL. The tz_dsttime field has never been used under Linux; it has not been and will not be supported by libc or glibc. Each and every occurrence of this field in the kernel source (other than the declaration) is a bug. Thus, the following is purely of historic interest.

The field tz_dsttime contains a symbolic constant (values are given below) that indicates in which part of the year Daylight Saving Time is in force. (Note: its value is constant throughout the year: it does not indicate that DST is in force, it just selects an algorithm.) The daylight saving time algorithms defined are as follows :


DST_NONE     /* not on dst */

DST_USA /* USA style dst */
DST_AUST /* Australian style dst */
DST_WET /* Western European dst */
DST_MET /* Middle European dst */
DST_EET /* Eastern European dst */
DST_CAN /* Canada */
DST_GB /* Great Britain and Eire */
DST_RUM /* Rumania */
DST_TUR /* Turkey */
DST_AUSTALT /* Australian style with shift in 1986 */

Of course it turned out that the period in which Daylight Saving Time is in force cannot be given by a simple algorithm, one per country; indeed, this period is determined by unpredictable political decisions. So this method of representing timezones has been abandoned. Under Linux, in a call to settimeofday() the tz_dsttime field should be zero.

Under Linux there are some peculiar "warp clock" semantics associated with the settimeofday() system call if on the very first call (after booting) that has a non-NULL tz argument, the tv argument is NULL and the tz_minuteswest field is non-zero. In such a case it is assumed that the CMOS clock is on local time, and that it has to be incremented by this amount to get UTC system time. No doubt it is a bad idea to use this feature.

Macros for operating on timeval structures are described in timeradd(3).  

RETURN VALUE

gettimeofday() and settimeofday() return 0 for success, or -1 for failure (in which case errno is set appropriately).  

ERRORS

EFAULT
One of tv or tz pointed outside the accessible address space.
EINVAL
Timezone (or something else) is invalid.
EPERM
The calling process has insufficient privilege to call settimeofday(); under Linux the CAP_SYS_TIME capability is required.
 

CONFORMING TO

SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeofday(). POSIX.1-2008 marks gettimeofday() as obsolete, recomending the use of clock_gettime(2) instead.  

NOTES

Traditionally, the fields of struct timeval were of type long.  

SEE ALSO

date(1), adjtimex(2), time(2), ctime(3), ftime(3), capabilities(7), time(7)  

COLOPHON

This page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO
COLOPHON

 

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